A new study reveals that precipitation could boost the iconic river’s flow in the next couple of decades despite the deleterious effects of warming temperatures due to climate change.
By Jane Palmer, AGU
Drought has plagued the Colorado River for the past 2 decades, but a recent study showed that the river’s flow could bounce back for the next 25 years due to increased precipitation.
The Colorado River stretches more than 2,400 kilometers from its Rocky Mountain headwaters in Wyoming and Colorado to the Gulf of California in Mexico and supplies water to residents, industry, and agriculture in seven states.
In the Southwest, however, the 2000–2021 period was the driest in the past 1,200 years. The multidecadal drought has driven the flows on the Colorado River to historic lows. Increasing temperatures due to climate change have led to concerns that the river’s flow will continue to decline.
“Nature has inherent variability in its precipitation patterns—at times these deliver abundant moisture, at other times the opposite.”
“The narrative that we read a lot about is that there’s no chance of recovery from our recent woes, as temperatures will keep warming and deplete more water,” said climate scientist Martin P. Hoerling at the University of Colorado Boulder, a coauthor of the study. “And we need to therefore confront a very dire outlook for the future.”
Hoerling’s previous research, however, indicated that precipitation, not temperature, has been the predominant factor affecting the flow in the Colorado River. A couple of decades of extremely low precipitation have been the main cause of the recent decline, Hoerling said. “Nature has inherent variability in its precipitation patterns—at times these deliver abundant moisture, at other times the opposite,” he said.
Stakeholders are beginning deliberations on post-2026 guidelines for managing future Colorado River water resources. And researchers have been working to parse the effects of both temperature and precipitation—their cyclical variations and trends—on this vital resource. “This includes understanding how much of our current drought is due to human influences versus natural swings,” Hoerling said. “Because that’s what is going to best inform if, and how, the river’s flow will recover.”
Good News and Bad News
To start answering these questions, the researchers compared precipitation and temperature data to water flow records at Lee Ferry in Arizona, the dividing point of the river’s Upper and Lower Basins, which date back to 1895. Lee Ferry is the historical gauge, and almost all the water in the river is generated above this point.
Their analysis found that 80% of the recent low flow, compared with the average flow measured at the beginning of the 20th century, was due to low precipitation and 20% was due to rising temperatures. “So again, it’s consistent with our view that historically, precipitation is driving the bus,” Hoerling said.
The team then used a collection of uniquely high-resolution climate models to simulate the river’s flow through 2050, assuming that temperatures will continue to rise. They assumed three different flow sensitivities to temperature increases.
“The standing narrative is a dark one based on temperature projections alone, and we are adding a bit of light to the story by presenting a more complete picture of recovery odds.”
The projections indicate a 70% chance of increased precipitation in the Upper Basin during 2026–2050. The magnitude of increases, relative to recent decades, could compensate for the negative effects of further warming, leading to a net increase in river flows, Hoerling said.
“The standing narrative is a dark one based on temperature projections alone, and we are adding a bit of light to the story by presenting a more complete picture of recovery odds,” he said.
The study’s findings also highlight the need for caution, however: The projections suggest a 4% chance that the annual flow of the river averaged for 2026–2050 could be as low as 10 million acre-feet—a 20% reduction from the current level. “If that were to come to pass, it would be unfortunate,” Hoerling said. “But if you were unprepared, it could be disastrous.”
Forewarned Is Forearmed
The team published a paper on their findings in the Journal of Climate. The study indicates that the future of the river is not entirely bleak but that both high and low flows are more likely in future decades.
“The message for the water community is that the drier ‘dries’ and the wetter ‘wets’ are all connected,” said coauthor Eric Kuhn, the retired general manager of the Colorado River Water Conservation District.
“The study is a good reminder to water managers and policymakers that there’s a wide range of possibilities for what is going to happen to Colorado River flow in the next 2 decades.”
“The study is a good reminder to water managers and policymakers that there’s a wide range of possibilities for what is going to happen to Colorado River flow in the next 2 decades,” said hydroclimatologist Park Williams from the University of California, Los Angeles, who was not involved with the study. “And the way that we choose to manage the river and allocate the river needs to be able to accommodate all of those possible future scenarios.”
The study presumes, however, that the effect of temperature on river flow and streamflow is relatively weak, which is an assumption that requires further investigation, Williams said. “How the Colorado River will be affected by global warming is so important that we need to continue beyond the analysis provided in this paper,” he said.
—Jane Palmer (@TJPalmerWrites), Science Writer
Citation: Palmer, J. (2024), Potential relief for the Colorado River’s near future, Eos, 105, https://doi.org/10.1029/2024EO240264. Published on 20 June 2024.
